“The reality is that the majority of farmworkers in the U.S. are foreign-born and unauthorized, which is well-known,” Feinstein wrote Tuesday, adding that she’s “afraid that this aggressive worksite enforcement strategy will deprive the agricultural sector of most of its workforce.”

Worksite monitoring has definitely heated up.

In 2008, the last year of the George W. Bush administration, U.S. Immigration and Customs Enforcement agents audited 503 companies nationwide for employee eligibility. Last year, ICE agents conducted more than 3,000 such audits.

In 2008, federal officials issued only 18 “final orders” fining employers and ordering them to stop breaking the law against hiring workers who are in the U.S. illegally. Officials collected $675,209 in fines that year. Last year, officials issued 495 final orders and collected $12.4 million in fines.

“In agriculture, they’ve been focusing on our processors and our packing facilities, and they are causing a tremendous amount of damage,” Manuel Cunha, the president of the Nisei Farmers League in Fresno, Calif., said in an interview Thursday.

The California farmers pressed the point several weeks ago with Sen. Marco Rubio, R-Fla., who was visiting Bakersfield, Calif., as part of a fundraising swing through the Golden State. Cunha said that the farmers, working with members of the U.S. House of Representatives from the Central Valley, were trying to raise similar concerns directly with Obama administration officials.

“They’re making these employers fire their workers,” Cunha said.

An Immigration and Customs Enforcement spokesman couldn’t be reached to comment Thursday, but on its website the agency underscored the importance of enforcement.

“Employment is a primary driving force behind illegal immigration,” the agency says. “Responsible employers who conduct their business lawfully are put at an unfair disadvantage when they try to compete with unscrupulous businesses. The unscrupulous businesses may gain a competitive edge by not paying their unauthorized workers prevailing wages and benefits.”

The farmers’ complaints, though, also show how tricky enforcement can become. While border security might sound like a no-brainer, employer-focused enforcement may bite the politically connected. Consequently, lawmakers can end up toggling between tough talk and calls for greater leeway.

“Elected officials from areas with concentrations of particular industries will represent those interests when they can,” former Immigration and Naturalization Service Commissioner Doris Meissner, who’s now with the Migration Policy Institute research center, said in an interview Thursday, adding that “this probably puts a lot more pressure on the executive branch and can narrow the room in which the executive branch has to move.”

Meissner recalled several instances from her own INS years when “very strong enforcement advocates” suddenly complained about constituents getting pinched, as when a law-and-order Georgia senator complained in the late 1990s about roundups that hindered the state’s Vidalia onion harvest.

Border security and interior enforcement play a significant role in the comprehensive immigration bill the Senate passed in June. The massive bill, some 1,920 pages when it passed the Senate Judiciary Committee, grew even larger when senators added more stringent enforcement measures.

The bill includes a provision that requires all employers to adopt the E-Verify employee verification system within five years.

In the House, Republican leaders have said they hope to move separate immigration-related bills this fall, potentially focusing on border security and E-Verify. In both the House and Senate, a common refrain is that comprehensive immigration legislation needs enforcement to balance potential leniency toward those in this country illegally.

“This is something where we need to have a top-to-bottom approach,” Rep. Jeff Denham of California, one of the House Republicans who now support a comprehensive bill, said during House debate several weeks ago..

The work-site monitoring that’s incited the backlash involves audits of the I-9 forms and accompanying documents that attest to an employee’s work eligibility. Monetary penalties for employers that knowingly hire workers who are in the U.S. illegally start at $375 and may go as high as $16,000 per violation.

Penalties for I-9 violations may range from $110 to $1,100 per violation. This can add up.

Last month, for instance, the 9th U.S. Circuit Court of Appeals upheld a $173,250 civil fine against a small Washington state firm called Ketchikan Drywall Services. An ICE audit concluded that the company had failed to maintain accurate and complete I-9 forms for myriad, mostly temporary employees.

“The penalties were imposed for substantive deficiencies,” Judge A. Wallace Tashima wrote for the court.

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